I gave a Canon 40D and a 580EXII flash a tough challenge yesterday, and it didn't do well.

There's a hummingbird feeder just outside my patio door; I'm in a warm place. I put the camera on a monopod, attached a 24-70 F:2.8 lens and a 580EXII flash with a DIY flash diffuser, and walked slowly toward the feeder until I was no more than 18 inches from it. I stood in position for a long time, and eventually enough curious and hungry birds started coming around. Before they arrived, I set the flash mode to manual and the power level to 1/4; I have found that the ETTL preflash spooks the birds.

By the time I got into position, the ambient light was pretty low, and I found that the camera's autofocus system was unable to focus on the birds when they hovered briefly above the feeder's plastic flower. Eventually I set the lens to manual focus and, using live view, set the focal point slightly behind the flower at a point where I guessed the birds might be in their hovering flight. I tried to focus manually on each bird, but they would hold positions only very briefly, and my results, although colorful, were not sharp, even with the lens set to F:11 to increase the depth of field. I realize that by being so close to my target, I multiplied my chances for focusing errors because the relative distances between the flash and the various focusing points were quite large.

Today, I remembered that Custom Function 8 on the 580EXII activates an AF-assist beam in low light. That feature is enabled on my two 580s, and I wondered why the beam wouldn't have helped me focus on the boards. I brought the camera fitted with a flash into a dark room and tried to see any evidence of that beam. I couldn't see it after several tests, though the photos looked pretty sharp.

I'm wondering if the AF-assist beam just wasn't working or whether there was anything I could have done with this camera and flash combination to produce more usable results. I'd appreciate your thoughts. Thanks!

I initially thought your problem may be being too close. The assist beam has a minimum range of about 2 feet.

If you aren't seeing the beam in a dark room though, it sounds like the beam may not be working. Make sure you are in "one shot" AF mode since the beam does not work in servo. If it is in "one shot" and still not working, check the assist beam settings both on the camera and on the flash. Beyond that, it may be a malfunctioning flash.

Even with it working, your results with small birds may be hit and miss. You will need to keep the bird on a focus point and it will need to be big enough relative to the sensor to reflect the assist beam cross-hatch.

Thanks, Eyeball; the mode setting was the problem. My dark-room test now shows the red beam. Of course, the birds may take off when they see it, so I'd have to disable it anyway. I will back up a bit until I reach the minimum AF-beam distance.